1. Field of the Invention
The present invention relates to a method for producing a water-soluble sulfonated product of polystyrene having a high purity and containing a small amount of by-products at a high yield.
2. Description of the Related Art
Various sulfonating methods of polystyrene have been proposed. For example, polystyrene is sulfonated by chlorosulfonic acid in a chlorinated hydrocarbon by adding one or more of water, sulfuric acid, hydrochloric acid, methyl alcohol, and ethyl alcohol as proposed in Japanese Examined Patent Publication (Kokoku) No. JP-B-50-33838, an anionic compound such as a polystyrene sulfonate and/or a nonionic compound such as methyl cellulose is added in the sulfonation reaction as proposed in Japanese Examined Patent Publication (Kokoku) No. JP-B-51-37226, or the hydrogen halide salt of a Lewis base (e.g., alkali halide such as NaCl) is added in the sulfonation reaction as proposed in Japanese Examined Patent Publication (Kokoku) No. JP-B-51-37227. These methods are all intended to obtain water-soluble polystyrene sulfonate containing a small amount of by-products at a high yield, while preventing the formation of crosslinking by --SO.sub.2 -- at the intermolecules or inner molecules of the polystyrene. However, the methods proposed in Nos. JP-B-50-33838 and JP-B-51-37226 have problems in that, since water is present in the reaction system, the water forms, together with the halogenated hydrocarbon solvent, a heterogeneous system and, therefore, it is difficult to uniformly disperse the water in the system and to maintain the homogeneous reaction system, and that, since the water is reacted with the sulfonating agent, the necessary amount of the sulfonating agent is unpreferable increased. On the other hand, the method disclosed in No. JP-B-51-37227 has problems in that, since the inorganic salt is used in the sulfonation reaction, the inorganic salt must be separated from the reaction mixture, after completing the sulfonation reaction, which is not effective as a practical process. Furthermore, the above-mentioned methods are not satisfactory from the viewpoints of, for example, the prevention of crosslinking at the intermolecules or innermolecules of polystyrene, and the yield. Especially, a serious problem occurs in the industrial scale in that the quantities of the sulfonation products are largely fluctuated.
On the other hand, the present inventors proposed in Japanese Patent Application No. 60-91038 that specified amounts of a benzene sulfonic acid and/or an alkylbenzene sulfonate having a C.sub.1 -C.sub.20 alkyl group be used, and that specified sulfonating be used in the sulfonation of polystyrene. However, although this technique can produce the desired sulfonated polystyrene at a yield higher than those of the above-mentioned prior art, there still remains a need to develop polystyrene sulfonation methods having a lower by-product content at a higher yield.